Closed Subscriber Group (CSG) cells are used to provide service to small numbers of users. For example, a “home NodeB” can have a small coverage area and provide access only to members of a home. CSG cells are used to provide better service to the user, e.g., higher data rates, and extend coverage of the operator's network. 3GPP is currently defining detailed behavior of CSG cells and the user equipment (UE) behavior in the presence of CSG cells.
CSG cells can be deployed within the coverage area of conventional macro cells and on the same frequency as the macro cells (mixed carrier). Unlike the deployment of the macro cells, detailed RF planning is not done for typical CSG cell deployments. Typically, a user installs a CSG cell at a home or office. Given that a CSG cell bars access to all except a small specific set of UEs, it is possible to have UEs close to the CSG cell that are not allowed to access to the CSG cell. Such UEs will normally be served by the macro cell. UL transmission from such macro cell UEs interferes with the UL transmissions of UEs in the CSG cell. This can cause service degradation for the UEs in the CSG cell. Such degradation may be characterized by dropped calls, lowered data throughput, etc. The interference can result in a higher noise floor in the CSG cell, causing the CSG cell UEs to increase their transmit power to overcome the interference, which in turn interferes with the UL transmission of the macro cell UEs, causing the macro cell UEs to increase their transmit power to overcome the interference, and so on.
3GPP RAN2 and RAN4 have discussed the problem and the following approach to avoid or reduce the interference is being investigated: A macro cell UE that can potentially cause interference to the UL transmissions of CSG cell UEs should be required to bar the frequency and reselect to a different frequency or a different Radio Access Technology (RAT). RAN2 and RAN4 have further discussed how this objective can be accomplished for UEs in the idle mode. In idle mode a UE is generally expected to camp on the “best cell”, i.e., the cell that is ranked highest in its reselection ranking. The UE therefore reads system information only from the highest ranked cell. If the highest ranked cell is not suitable to the UE, the UE bars the frequency (all cells on the frequency) for a specified duration.
With the above behavior, in the situation where a UE is near a CSG cell (i.e., the CSG cell is the highest ranked) that the UE is not allowed to access, the UE must reselect to a different frequency. This leads to unnecessary reselections. The following method has been proposed for controlling the reselections: An Intra-frequency reselection indication (IFRI) is transmitted by every CSG cell in its system information message. This indication can be “allowed” or “disallowed”. If an idle mode UE finds a CSG cell to be the highest ranked cell, it reads the IFRI from the system information. If the IFRI is set to “disallowed”, the UE bars the frequency (behavior is same as when highest ranked cell is not suitable). If the IFRI is set to “allowed”, the UE camps on the second highest ranked cell (the macro cell).
The above method can have a substantial battery life impact on a UE as it requires the UE to read system information of those CSG cells that it is not allowed to access via its transceiver. In urban areas there can be numerous CSG cells, which can have a severely adverse effect on battery life. Furthermore, setting the IFRI “correctly” is difficult. If the typical setting of the IFRI is “disallowed” it may very frequently cause a UE to reselect to other frequencies or RATs. If the typical setting of IFRI is “allowed”, the UE can remain on the frequency and read system information of CSG cells, causing a significant battery drain.
A modification of the above IFRI method is considered in 3GPP document R4-082384, where it is proposed to signal a differential path-loss threshold from the CSG cell along with the IFRI. A UE computes the difference in measured path-loss between the macro cell and the CSG cell, and if the difference is higher than the signaled path-loss threshold, it obeys the IFRI from the CSG cell. Since such a threshold would have to be signaled by the CSG cell in a system information message, this method also requires the UE to read system information of those CSG cells that the UE is not allowed to access.
The various aspects, features and advantages of the disclosure will become more fully apparent to those with ordinary skill in the art, on careful consideration of the following Detailed Description and the accompanying drawings. The drawings have been simplified for clarity and are not necessarily drawn to scale.